Microphone arrangement

ABSTRACT

The present invention relates to a microphone arrangement (M) which has a charge pump (LP), which produces a DC voltage, a transducer (WA), which converts acoustic signals into electrical signals and which is connected to the charge pump (LP), and a control unit (VCLFS), which controls the charge pump (LP) and which adjusts the DC voltage produced by the charge pump (LP).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Stage of International ApplicationNo. PCT/EP2012/050978, filed Jan. 23, 2012, which claims the benefit ofGermany Patent Application No. 10 2011 010 506.9, filed on Feb. 7, 2011,both of which are incorporated herein by reference in their entireties.

The invention relates to a microphone arrangement which has a transducerfor converting acoustic signals into electrical signals and canaccordingly be used as a microphone.

MEMS microphones can be used in mobile telephones. It is important herethat the MEMS microphone is operational directly after it is switchedon. Start-up times of less than 50 ms are the aim.

A transducer in an MEMS microphone has a capacitive element, wherein avoltage is applied between a fixed and a moving electrode. After themicrophone is switched on, said voltage needs to be built up as quicklyas possible. Furthermore, said voltage can be higher than the voltagewhich is produced by a battery for supplying power to the mobiletelephone. For this reason, so-called charge pumps, which apply thedesired voltage between the electrodes of the transducer, are used.

WO 2009/135815 A1 describes a charge pump which can be connected to anelectro-acoustic transducer in an MEMS microphone. The circuit describedthere is configured in such a way that, if too high a voltage isproduced by the charge pump, a current flows away into a regulablecurrent source and the charge pump does not reach its nominal voltage.However, the circuit exhibits voltage fluctuations at the output sincethe operating point of the charge pump is on a current/voltage edge.

It is now the object of the present invention to provide a microphonearrangement which is more stable and in which the voltage applied to thetransducer can be built up in a stable manner within a short time.

This object is achieved by means of a microphone arrangement having thefeatures of claim 1. Advantageous configurations of the invention emergefrom further claims.

According to the invention, a microphone arrangement is proposed whichhas a charge pump, which produces a DC voltage, a transducer, whichconverts acoustic signals into electrical signals and which is connectedto the charge pump, and a control unit, which controls the charge pumpand which adjusts the DC voltage produced by the charge pump.

A charge pump is an electrical circuit to which an input voltage can beapplied and which produces an electrical output voltage which is higherthan the input voltage, wherein the input and output voltages are DCvoltages. In the case of a charge pump, the output voltage is producedby means of capacitors and by periodic toggling of switches. A chargepump may also have diodes.

The control unit makes it possible, according to the invention, toadjust the DC voltage produced by the charge pump. For this purpose, thecontrol unit can vary the level of the input voltage which is applied tothe charge pump and/or the frequency at which switches in the chargepump are toggled. If a higher input voltage is applied, this leads to ahigher output voltage from the charge pump. If the toggling frequency inthe charge pump is increased, a rise time, in which the voltage in thecharge pump is built up, is reduced as a result.

In one embodiment, the maximum DC voltage able to be produced by thecharge pump is larger than the optimum operating voltage of thetransducer. The optimum operating voltage V_(BIAS) is the voltage forwhich the transducer has been designed. Accordingly, after themicrophone is switched on, firstly a maximum producible DC voltage canbe produced by the charge pump, which DC voltage ensures that thevoltage applied to the transducer is built up very quickly. In a secondphase after the microphone is switched on, the DC voltage produced bythe charge pump can be lowered to an absolute value which is lower thanthe absolute value of the maximum producible DC voltage. In this way,the time for applying the voltage to the transducer can be minimized.

The transducer is designed for an optimum operating voltage V_(BIAS).Preferably, in the second phase the charge pump produces a DC voltagewhich corresponds to said voltage V_(BIAS). The optimum operatingvoltage of the transducer is substantially predefined by the geometry,the material properties and an impressed mechanical re-tensioning of thetwo electrodes.

Various possibilities are conceivable for controlling the charge pump bymeans of the control unit. These include a continuous feedback circuit,a single feedback circuit and a time-based circuit.

For a continuous feedback circuit, the microphone arrangement can alsohave a voltage divider, a reference voltage source and a differentialamplifier.

The voltage divider can be linked to that path which interconnects thecharge pump with the transducer. The voltage divider can be a seriescircuit of diodes or a series circuit of capacitors, for example.Alternatively, the voltage divider can have a plurality of diodesconnected in series and also a plurality of capacitors connected inseries, wherein the diodes are arranged in a first path and thecapacitors are arranged in a second path, which is connected in parallelwith the first path.

The reference voltage source can produce a DC voltage with a fixedabsolute value. Said DC voltage multiplied by a pump factor correspondsto the optimum operating voltage V_(BIAS) of the transducer.

Furthermore, the differential amplifier can be interconnected with thereference voltage source and with the voltage divider. The voltagedivider supplies a voltage which is proportional to the voltage producedat present by the charge pump. Now the differential amplifier outputs asignal which is proportional to the difference between the voltageproduced at present by the charge pump and the optimum operating voltageV_(BIAS).

The output signal from the differential amplifier can be applied to thecontrol unit. Accordingly, the control unit continually receivesfeedback via the differential amplifier, which feedback indicateswhether the voltage produced by the charge pump is too high or too low.The control unit can continuously adapt the voltage produced by thecharge pump in accordance with said feedback. For this purpose, thecontrol unit regulates the level of the input voltage for the chargepump and/or the frequency at which the charge pump is operated. A higherinput voltage leads to a higher output voltage. If the frequency of thecharge pump is increased, the rise time is reduced as a result.

For a feedback circuit single, the microphone arrangement also has atrigger in addition to the voltage divider, the reference voltage sourceand the differential amplifier. The two inputs of the differentialamplifier can be connected to the voltage divider and the referencevoltage source. Accordingly, the differential amplifier produces asignal which is proportional to the difference between the voltageapplied to the microphone at present and the optimum operating voltage.Said signal is output at the output of the differential amplifier andsaid output can be connected to the trigger.

Moreover, the trigger can be connected to the control unit and to atleast one switch. In one exemplary embodiment, the switch connects thevoltage divider to the connecting path from charge pump to transducer ina closed state. In an open state, the switch decouples the voltagedivider from the charge pump and the transducer.

If the voltage divider is decoupled from the charge pump, then noleakage currents can flow away via the voltage divider and the chargepump can be operated with low losses. In contrast to WO 2009/135815, inwhich the voltage divider is always interconnected with the charge pump,here the voltage divider does not contribute to fluctuations insensitivity in the microphone. In this way, a substantial disturbancemechanism can be eliminated and stable operation of the microphonearrangement can be ensured.

In one embodiment, the trigger opens the switch as soon as the signalproduced by the differential amplifier reaches a threshold value. Saidreaching of the threshold value corresponds to the transition from thefirst phase, in which a maximum DC voltage is produced by the chargepump, to a second phase, wherein, in the second phase, a DC voltage isproduced by the charge pump, which DC voltage is smaller than themaximum DC voltage able to be produced by the charge pump. The controlunit is likewise connected to the trigger and adjusts the voltageproduced by the charge pump depending on the signal which is output bythe differential amplifier via the trigger.

In a third embodiment of the present invention, a time-based circuit isselected. This can have means for measuring the time which has elapsedsince the microphone arrangement was switched on. Preferably, the meansfor measuring the time are connected to the control unit. After astipulated time, the control unit reduces the voltage produced by thecharge pump from a maximum value to a value which is lower than themaximum value and is preferably equal to V_(BIAS).

Furthermore, the charge pump and the transducer can be connected to oneanother via a high-impedance circuit. Said high-impedance circuit canhave a pair of cross-coupled diodes. The oppositely connected diodeshave an impedance in the range of TΩ during operation of the microphone.In this way, it is ensured that the capacitor of the transducer cannotbe discharged during operation and therefore remains at a constantcharge.

The transducer can also be connected to a path in which a capacitor isconnected in series with an amplifier. An electrical signal, which thetransducer has converted from an acoustic signal, can be transmitted toan input and output unit of the microphone arrangement via said path.

In one exemplary embodiment, the transducer has a fixed electrode and amoving electrode, for example a diaphragm. The charge pump produces avoltage which is applied between said two electrodes.

The invention will be explained in more detail below on the basis ofexemplary embodiments and the associated figures. The figures showvarious exemplary embodiments of the invention on the basis of schematicillustrations which are not drawn to scale.

FIG. 1 shows a first exemplary embodiment of a microphone arrangement ofthe invention.

FIG. 2 shows a second exemplary embodiment of a microphone arrangementof the invention.

FIG. 3 shows the second exemplary embodiment in a second state.

FIG. 4 shows a second variant of the second exemplary embodiment in afirst state.

FIG. 5 shows the second variant in a second state.

FIG. 1 shows a first exemplary embodiment of the microphone arrangementM of the invention. This is a continuous feedback circuit.

The microphone arrangement M has a transducer WA which converts acousticsignals into electrical signals. Said transducer WA has a capacitor witha fixed and a moving electrode. A bias voltage is applied between thetwo electrodes. The capacitor is designed for an optimum operatingvoltage V_(BIAS).

The microphone arrangement M also has a charge pump LP. This may be aDickson charge pump. The charge pump LP is connected to a voltage sourceSQ, which provides a supply voltage, via a two-phase clock 2PC.

The charge pump LP and the transducer WA are connected to one anothervia a first path P1.

The input of the charge pump LP is connected to the two-phase clock 2PC,the latter in turn being connected to a variable clock level andfrequency shifter VCLFS. In this case, the variable clock level andfrequency shifter VCLFS forms the control unit. The output signal fromthe variable clock level and frequency shifter VCLFS is transmitted asinput signal to the two-phase clock 2PC. The two-phase clock 2PC nowproduces a signal which is used to control the charge pump LP. Thesignal level and the frequency of said signal are adjusted by means ofthe output signal from the variable clock level and frequency shifterVCLFS.

Furthermore, the two-phase clock 2PC is connected to an oscillator OS.

The charge pump LP is configured in such a way that it produces anoutput voltage which is larger than the applied input voltage. Inparticular, the output voltage can be larger than the supply voltage forthe microphone arrangement M that is produced by a battery. The absolutevalue of the output voltage which is produced by the charge pump LP isdetermined by the variable clock level and frequency shifter VCLFS. Ifthe signal from the variable clock level and frequency shifter VCLFS isincreased in amplitude or frequency, this leads to a larger outputvoltage.

The voltage which is output by the charge pump LP is applied to a firstelectrode of the transducer WA. The second electrode of the transducerWA is connected to the voltage source SQ. Accordingly, the secondelectrode of the transducer WA is at a lower voltage level and apotential difference is produced between the two electrodes of thetransducer WA. This makes it possible to determine a change incapacitance in the transducer WA.

The charge pump LP is interconnected with the transducer via twooppositely connected diodes D1 and D2. The first path P1 splits into twoparallel subpaths UP1 and UP2, wherein in each case one of the twooppositely connected diodes D1 and D2 is arranged in each subpath UP1and UP2. These two subpaths UP1 and UP2 form a high-impedance element,the impedance of which is—in a steady state—in the teraohmic range. Inthis way, it is ensured that the transducer WA is not discharged andthat almost no current flows between the charge pump LP and thetransducer WA.

The first path P1, which connects the transducer WA and the charge pumpLP to one another, also has three nodes K1, K2 and K3.

A second path P2, in which a coupling capacitor C1 and an amplifier VERare connected in series, branches off from the first node K1. The secondpath P2 is connected to an input and output port I/O of the microphonearrangement M.

The second node K2 is connected to a third path P3, which has a voltagedivider ST. The voltage divider ST here is a series circuit of diodes.Alternatively, it may also be a series circuit of capacitors.

A second capacitor C2 is arranged in a fourth path P4 in parallel withthe voltage divider. The fourth path P4 is connected to the node K2 ofthe first path. The second capacitor C2 serves to filter ahigh-frequency voltage component which originates from the charge pump.

The third and fourth paths P3 and P4 are also linked together via acommon node K4 and connected to ground.

The voltage divider ST is connected to a first input of a differentialamplifier DV. The second input of the differential amplifier DV isconnected to a reference voltage source RSQ. The reference voltagesource RSQ produces a constant DC voltage which is proportional to theoptimum operating voltage V_(BIAS) of the transducer.

The differential amplifier DV also has an output. The signal which isoutput at this output is proportional to the difference between the twovoltages applied in each case to an input of the differential amplifierDV.

The voltage which is proportional to the voltage produced at present atthe charge pump LP is applied via the voltage divider ST to the firstinput of the differential amplifier DV. A voltage which is proportionalto the optimum operating voltage V_(BIAS) is applied to the secondinput. Accordingly, the output signal from the differential amplifier DVindicates by what value the voltage produced at present deviates fromthe optimum operating voltage V_(BIAS). The mathematical sign of theoutput voltage indicates whether the voltage produced at present or theoptimum operating voltage is larger in terms of absolute value.

The output of the differential amplifier DV is in turn connected to thevariable clock level and frequency shifter VCLFS. Accordingly, thedifferential amplifier DV provides feedback to the variable clock leveland frequency shifter VCLFS. The variable clock level and frequencyshifter now adapts its output signal such that the voltage produced bythe charge pump LP is adapted to suit the reference voltage. If, forexample, the voltage produced by the charge pump LP is smaller than thedesired optimum voltage V_(BIAS), the amplitude of the output signalfrom the variable clock level and frequency shifter VCLFS is increasedand thus the voltage of the charge pump LP is increased. Conversely, thevoltage from the charge pump LP can also be reduced by means of adecrease in amplitude and.

Said adaptation is repeated continuously during operation of themicrophone arrangement M. In this way, it can be ensured that the outputvoltage produced by the charge pump LP corresponds to the optimumoperating voltage V_(BIAS).

Almost no current flows through the voltage divider ST and the secondcapacitor C2. In this way, it is ensured that the charge pump LP can beoperated at high impedance. Accordingly, the operating point of themicrophone arrangement M is on a flat plateau in a current/voltagecharacteristic. Thus, the microphone arrangement M operates in a stablesensitivity range.

FIG. 2 shows a second exemplary embodiment of the present invention.This is a single feedback circuit.

In contrast to the circuit shown in FIG. 1, the output of thedifferential amplifier DV is not directly connected to the variableclock level and frequency shifter VCLFS but rather is connected to atrigger TRI. The circuit also has two switches S1 and S2, which arearranged in the third path P3, in which the voltage divider is arranged.In the third path P3, a first switch S1 is arranged between the diodesof the voltage divider ST and the node K2, which connects the third pathP3 and the first path P1 to one another. The second switch S2 isarranged between the diodes of the voltage divider ST and the furthernode K4, which connects the third path to ground and to the fourth path.

The trigger TRI is connected to the two switches S1 and S2 and is usedas control element for the two switches S1 and S2. Accordingly, thetrigger TRI can open and close the switches S1 and S2. The trigger TRIis also connected to the variable clock level and frequency shifterVCLFS.

The output signal from the differential amplifier DV is in turnproportional to the difference between the voltage produced at presentby the charge pump and the optimum operating voltage V_(BIAS). If saidoutput signal exceeds a predefined limit value, the trigger TRI opensthe two switches S1 and S2. Said limit value is typically reached whenthe two voltages are equal.

The trigger TRI also transmits the output signal from the differentialamplifier DV to the variable clock level and frequency shifter VCLFS.Said variable clock level and frequency shifter in turn controls theoutput voltage of the charge pump LP.

In a first phase following after the microphone arrangement M isswitched on, the charge pump LP produces the maximum possible DC voltagefor charging the capacitor of the transducer WA. If the present voltagenow corresponds to the optimum operating voltage V_(BIAS), the triggerTRI opens the switches S1 and S2. At the same time, the variable clocklevel and frequency shifter VCLFS regulates the output voltage of thecharge pump LP to a lower level, namely that of the optimum voltageV_(BIAS).

FIG. 3 shows the exemplary embodiment shown in FIG. 2 in the secondphase with open switches S1 and S2.

Now the third path P3, which has the voltage divider ST, is decoupledfrom the first path P1, which forms the connection between charge pumpLP and transducer WA. Accordingly, no current can flow away via thevoltage divider ST and influence the charge of the transducer WA.

FIGS. 4 and 5 show a second variant of this single feedback circuit.Here, the voltage divider ST, which has a series circuit of diodes inthe first variant, is replaced by a voltage divider ST which has aseries circuit of capacitors. The functional principle of the circuitcorresponds to the functional principle which has already been explainedin connection with FIGS. 2 and 3.

A further possibility for configuring the microphone arrangement Minvolves a circuit which completely dispenses with feedback and insteaduses a time-based method. A circuit such as this is not illustrated inthe figures. In the case of the time-based method, the circuit has meansfor determining the time since the microphone arrangement M was switchedon. The variable clock level and frequency shifter VCLFS controls thecharge pump LP with a signal, wherein the amplitude and/or frequency ofthe signal are dependent on the time.

In a first phase after the microphone arrangement M is switched on, amaximum output voltage from the charge pump LP is produced. After afirmly predefined time, the output voltage of the charge pump LP isreduced to the value of the optimum operating voltage V_(BIAS).

The time-based method has the advantage that the circuit issignificantly simplified. It is possible to dispense with a feedbackloop.

The continuous or single feedback circuits shown in the first and secondexemplary embodiments have the advantage that the charge pump LP can becontrolled in a more targeted and stable manner. Any manufacturingtolerances in the individual components can be compensated for.

All three embodiments are characterized by a quick and stable build-upof the voltage at the transducer WA.

LIST OF REFERENCE SIGNS

-   M—microphone arrangement-   WA—transducer-   LP—charge pump-   SQ—voltage source-   VCLFS—variable clock level and frequency shifter-   2PC—two-phase clock-   OS—oscillator-   D1, D2—diode-   P1-P4—first-fourth paths-   K1-K4—nodes-   UP1, UP2—subpath-   C1—coupling capacitor-   C2—second capacitor-   VER—amplifier-   I/O—input and output port-   ST—voltage divider-   DV—differential amplifier-   RSQ—reference voltage source-   TRI—trigger-   S1, S2—switch

The invention claimed is:
 1. A microphone arrangement having: a charge pump, which produces a DC voltage, a transducer, which converts acoustic signals into electrical signals and which is connected to the charge pump, a control unit, which controls the charge pump and which adjusts the DC voltage produced by the charge pump, and a differential amplifier configured to output a signal proportional to the difference between a voltage produced at present by the charge pump and an optimum operating voltage of the transducer, wherein the output signal from the differential amplifier is applied to the control unit, and wherein the control unit is configured to adapt the voltage produced by the charge pump in accordance with the output signal from the differential amplifier.
 2. The microphone arrangement according to claim 1, in which the control unit regulates the input voltage of the charge pump and/or the frequency at which the charge pump is controlled.
 3. The microphone arrangement according to claim 1, in which the maximum DC voltage able to be produced by the charge pump is larger than the optimum operating voltage of the transducer.
 4. The microphone arrangement according to claim 1, in which the control unit can vary the absolute value and/or the frequency of a voltage applied to the charge pump.
 5. The microphone arrangement according to claim 1, wherein, in a first phase after the microphone is switched on, the control unit controls the charge pump in such a way that the charge pump produces a maximum DC voltage.
 6. The microphone arrangement according to claim 5, wherein, in a second phase following the first phase, the control unit controls the charge pump in such a way that the charge pump produces a DC voltage the absolute value of which is lower than the absolute value of the maximum DC voltage.
 7. The microphone arrangement according to claim 1, also having a voltage divider, which is interconnected with the charge pump and the transducer, a reference voltage source, which produces DC voltage with a fixed absolute value, and a differential amplifier with two inputs, which outputs a signal that is proportional to the difference in the voltage being applied to the two inputs, wherein the first input is connected to the voltage divider, the second input is connected to the reference voltage source and an output is connected to the control unit.
 8. The microphone arrangement according to claim 7, wherein the control unit adjusts the voltage produced by the charge pump depending on the signal which is output by the differential amplifier.
 9. The microphone arrangement according to claim 7, wherein the voltage divider has a plurality of diodes or capacitors connected in series.
 10. The microphone arrangement according to claim 7, wherein the voltage divider has a plurality of diodes connected in series and also has a plurality of capacitors connected in series, wherein the diodes are arranged in a first path and the capacitors are arranged in a second path, which is connected in parallel with the first path.
 11. The microphone arrangement according to claim 1, also having a voltage divider, which is interconnected with the charge pump and the transducer, a reference voltage source, which produces DC voltage with a fixed absolute value, a trigger, which is connected to the control unit and to a switch, wherein the switch interconnects the voltage divider with the charge pump and the transducer in a closed state and decouples the voltage divider from the charge pump and the transducer in an open state, and a differential amplifier with two inputs, which outputs a signal that is proportional to the difference in the voltage being applied to the two inputs, wherein the first input is connected to the voltage divider, the second input is connected to the reference voltage source and an output is connected to the trigger.
 12. The microphone arrangement according to claim 11, wherein the trigger opens the switch when the signal produced by the differential amplifier reaches a threshold value.
 13. The microphone arrangement according to claim 11, wherein the control unit adjusts the voltage produced by the charge pump depending on the signal which is output by the differential amplifier.
 14. The microphone arrangement according to claim 11, wherein the voltage divider has a plurality of diodes connected in series and also has a plurality of capacitors connected in series, wherein the diodes are arranged in a first path and the capacitors are arranged in a second path, which is connected in parallel with the first path.
 15. The microphone arrangement according to claim 1, also having means for measuring the time since the microphone arrangement was switched on, wherein the means for measuring the time are connected to the control unit.
 16. The microphone arrangement according to claim 15, wherein the control unit adjusts the voltage produced by the charge pump depending on the time since the microphone arrangement was switched on.
 17. The microphone arrangement according to claim 1, wherein the charge pump and the transducer are connected to one another via a high-impedance circuit.
 18. The microphone arrangement according to claim 17, wherein the high-impedance circuit has a pair of cross-coupled diodes.
 19. The microphone arrangement according to claim 1, wherein the transducer is also interconnected with a capacitor and an amplifier.
 20. The microphone arrangement according to claim 1, wherein the transducer has a fixed electrode and a moving electrode, and wherein the voltage produced by the charge pump is applied between said two electrodes. 